Base station, radio communications system, base station control method, radio communications method and base station control program

ABSTRACT

[Problems] Because it is not possible to reduce sufficiently the number of base stations that starts uselessly in spite of being in low traffic state, it is not possible to lower interference between adjacent cells and power consumption sufficiently. 
     [Measures] A base station starts transmission of a control signal with predetermined power when communication between other base station and a mobile station is started and a first predetermined condition is satisfied.

TECHNICAL FIELD

The present invention relates to a base station, a radio communicationssystem, a base station control method, a radio communications method anda base station control program.

BACKGROUND ART

As a mobile communications system, for example, a cellular system isknown. A cellular system is a system to secure a communication range ofa service area for a wide range by laying out cells (communication areaof base stations which cover from several hundreds meters to severalkilometers). By the way, in case of a cellular system, in order to avoidthe situation where communication is impossible due to dark place or anincrease of number of terminals in a service area, countermeasures whichincrease the number of the base stations are being performed. In thatcase, there are variously arrangement patterns of the base stations (forexample, a case where a cover area of a certain cell overlaps with acover area of a cell which is adjacent to the certain cell, a case whereentire cover area of the certain cell is included in a cover area ofother cell, and a case where each cover area of three or more cellsoverlaps).

However, the overlap and the inclusion of the cover area might cause thegeneration of radio wave interference between the base stations. Radiointerference brings about lowering of channel capacity. Also, with anincrease of number of base stations, in spite of a mobile station notexisting in a cover area of the cell (that is, in spite of not beingused at all), probability rises that a base station which is keptstarted exists. Electric power is wasted by a base station which isbeing started wastefully and without being used at all.

Accordingly, technologies aiming at interference avoidance or powersavings in a mobile communications system are proposed. For example,Japanese Patent Application. Laid-Open No. 2003-37555 describes atechnology in which a certain radio base station monitors a transmissionsignal transmitted from other radio base stations, and by consideringtraffic status and received power of other radio base stations, suspendstransmission or starts transmission of own base station. The publicationmentions that the number of base stations operating in low traffic andthe interference in surrounding base stations are decreased. Further,the publication discloses a base station which, when traffic of otherbase stations is heavy, makes own base station return from a sleep stateto a normal state, and covers at least one mobile station which cannotbe accommodated by other stations in heavy traffic.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In the technology of the publication, own base station returns from thesleep state to the normal state only on the condition that traffic of anadjacent base station becomes heavy. However, even if heavy traffic isimposed on the adjacent base station, a mobile station which should behanded over from the adjacent base station to own base station may notexist actually. Accordingly, the base station of the publication has aconcern that it returns to the normal state wastefully even though theredoes not exist a mobile station which should be handed over from theadjacent base station to own base station and as a result, consumeselectric power wastefully. That is, the technology of the publicationcannot reduce sufficiently the number of base stations which has beenstarted wastefully in spite of the low traffic state. In consequence,the decreases of interference between adjacent cells and powerconsumption are insufficient.

An object of the present invention is to provide a base station, a radiocommunications system, a base station control method, a radiocommunications method and a base station control program capable ofsuppressing consumption of electric power of a base station and avoidingradio interference between base stations.

Measures for Solving the Problems

A base station of the present invention starts transmission of a controlsignal with predetermined power when communication between other basestation and a mobile station is started and a first predeterminedcondition is satisfied.

Also, a radio communications system of the present invention includes afirst base station, a second base station, and at least one mobilestation which can communicate with the first base station and the secondbase station; and the second base station starts transmission of acontrol signal with predetermined power when communication between thefirst base station and the mobile station is started and a predeterminedcondition is satisfied.

Also, in a base station control method of the present invention, a basestation around the other base station starts transmission of a controlsignal with predetermined power when communication between other basestation and a mobile station is started and a predetermined condition issatisfied.

Also, in a radio communications method of the present invention, asecond base station starts transmission of a control signal withpredetermined power when communication between a first base station anda mobile station is started and a predetermined condition is satisfied.Also, a base station control program of the present invention makes acomputer of base station around other base station execute processingwhich starts transmission of a control signal with predetermined power,when communication between the other base station and a mobile stationis started and a predetermined condition is satisfied.

Also, a mobile station of the present invention is the mobile stationwhich can communicate with a first base station and a second basestation, and receives a control signal transmitted from the second basestation with predetermined power when communication between the firstbase station and the mobile station is started and a predeterminedcondition is satisfied.

Effect of the Invention

According to the present invention, consumption of electric power of abase station is suppressed and radio interference between base stationsis avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A block diagram showing an example of a radio communicationssystem according to the first exemplary embodiment of the presentinvention

[FIG. 2] A block diagram showing an example of a first base station ofthe first exemplary embodiment

[FIG. 3] A block diagram showing an example of a second base station ofthe first exemplary embodiment

[FIG. 4] An explanatory drawing about state change of the second basestation

[FIG. 5] A sequence chart showing an example of operation of the radiocommunications system, when the second base station changes from anactive state to a radio transmission suspension state

[FIG. 6] A flow chart illustrating an example of operation, when thesecond base station changes from the active state to the radiotransmission suspension state

[FIG. 7] A sequence chart showing an example of operation of the radiocommunications system in the first exemplary embodiment on the occasionwhen the second base station changes from a radio transmissionsuspension state to an active state and hands over a mobile stationwhich are accommodated in the first base station to the second basestation

[FIG. 8] A flow chart showing an example of operation of the second basestation in the first exemplary embodiment on the occasion when thesecond base station changes from a radio transmission suspension stateto an active state and hands over a mobile station which areaccommodated the first base station to the second base station

[FIG. 9] A block diagram showing an example of a first base station ofthe second exemplary embodiment

[FIG. 10] A sequence chart showing an example of operation of the radiocommunications system in the second exemplary embodiment on the occasionwhen the second base station changes from a radio transmissionsuspension state to an active state and hands over a mobile stationwhich are accommodated in the first base station to the second basestation

[FIG. 11] A block diagram showing another example of a radiocommunications system according to the second exemplary embodiment ofthe present invention

[FIG. 12] A block diagram showing an example of a radio communicationssystem according to the third exemplary embodiment of the presentinvention

[FIG. 13] A sequence chart showing an example of operation of the radiocommunications system in the third exemplary embodiment on the occasionwhen the second base station changes from a radio transmissionsuspension state to an active state and hands over a mobile stationwhich are accommodated in the first base station to the second basestation

[FIG. 14] A figure describing a first method for carrying out handoverprocessing of a control signal at a communication stage, and a sequencechart showing an example of operation of a radio communications systemon the occasion when the second base station changes from a radiotransmission suspension state to an active state and hands over a mobilestation which are accommodated in the first base station to the secondbase station in the third exemplary embodiment

[FIG. 15] A figure describing a second method for carrying out handoverprocessing of a control signal at a communication stage, and a sequencechart showing an example of operation of a radio communications systemon the occasion when the second base station changes from a radiotransmission suspension state to an active state and hands over a mobilestation which are accommodated in the first base station to the secondbase station in the first exemplary embodiment

[FIG. 16] A figure describing a third method for carrying out handoverprocessing of a control signal at a communication stage, and a sequencechart showing an example of operation of a radio communications systemon the occasion when the second base station changes from a radiotransmission suspension state to an active state and hands over a mobilestation which are accommodated the first base station to the second basestation in the first exemplary embodiment

DESCRIPTION OF CODE

1, 4, 5 Base station (first base station)

2, 2-1, 2-2 Base station (second base station)

11-17 Cell of base station

100-109 Mobile station

200 RNC

308 Load management unit

358 State change control unit

360 Power control unit

450 Location information acquisition unit

Exemplary Embodiments of the Invention

In the following, exemplary embodiments of the present invention aredescribed in detail with reference to drawings.

A second base station according to an exemplary embodiment of thepresent invention starts transmission of a control signal (inparticular, a common control signal which will be broadcasted to thewhole area of the cell) based on a start request which is transmittedfrom a predetermined equipment (for example, a radio network controlleror a first base station), when communication between the first basestation and a mobile station is started and a predetermined condition issatisfied.

Further, in each of the following exemplary embodiments, a case where a“pilot signal” which is a common control signal transmitting a signal ofa predetermined pattern continually and repeatedly is employed as anexample of the “control signal” is described.

First Exemplary Embodiment

FIG. 1 is a block diagram showing an example of a radio communicationssystem according to the first exemplary embodiment of the presentinvention. This radio communications system includes a base station 1 (afirst base station), a base station 2 (a second base station), a mobilestation 100, a mobile station 101 and a Radio Network Controller 200(hereinafter, referred to as “RNC”). The base station 1 transmits thepilot signal to the mobile station in a cell 11. The mobile station 100and the mobile station 101 which received the pilot signal form a radiolink 1100 and a radio link 1101 respectively and communicate with thebase station 1. Similarly, the base station 2 can transmit the pilotsignal to the mobile station in a cell 12. The mobile station 100 whichreceived the pilot signal can form a radio link 1200 and communicatewith the base station 2. Here, a part at least overlaps between the cell11 and the cell 12. The RNC 200 is connected with the base station 1through a line 2001 and is also connected with the base station 2through a line 2002. The RNC 200 manages the base station 1 and the basestation 2. Here, the line 2001 and the line 2002 may be either a cableline or a wireless line, and is described as the cable line in thefollowing description.

FIG. 2 is a block diagram showing an example of the base station 1 asthe first base station shown in FIG. 1. The base station 1 includes anetwork communication unit 300, a RF (Radio Frequency) unit 302, areception signal processing unit 304, a transmission signal processingunit 306, a load management unit 308 and an antenna 310. The networkcommunication unit 300 performs cable communication with the RNC 200.The RF unit 302 performs radio communication with the mobile station100. The reception signal processing unit 304 processes a signalreceived from the mobile station 100 via the RF unit 302. Thetransmission signal processing unit 306 processes a signal fortransmitting to the mobile station 100 and transmits the signal to theRF unit 302. The load management unit 308 acquires communication trafficof the mobile station 100 which the base station 1 supports and numberof the mobile stations 100 in the cell 11 from the reception signalprocessing unit 304 and the network communication unit 350 as load data.

FIG. 3 is a block diagram showing an example of the base station 2 asthe second base station shown in FIG. 1. The base station 2 includes anetwork communication unit 350, a RF unit 352, a reception signalprocessing unit 354, a transmission signal processing unit 356, a statechange control unit 358, a power control unit 360, an antenna 362 and aload management unit 364. The network communication unit 350 performscable communication with the RNC 200. The RF unit 352 performs radiocommunication with the mobile station 100. The reception signalprocessing unit 354 processes a signal received from the mobile station100 via the RF unit 352. The transmission signal processing unit 356processes a signal for transmitting to the mobile station 100 andtransmits the signal to the RF unit 352. The load management unit 364acquires communication traffic of a mobile station which the basestation 2 supports and number of the mobile stations in the cell 12 fromthe reception signal processing unit 356 and the network communicationunit 350 as load data, and determines a presence of load in the basestation 2. Also, the load management unit 364 inquires of the RNC 200via the network communication unit 350 a received power status (whetherthe received power concerned exceeds a threshold value) of a pilotsignal of the base station 2 in a mobile station (for example, mobilestation 101 in FIG. 1) which exists around cell 12. The state changecontrol unit 358 controls, a change of operation states of the basestation 2 according to the instructions or the information from thenetwork communication unit 350, the reception signal processing unit 354or the load management unit 364. The power control unit 360 executeselectrical power control (for example, control of power ON/OFF) of thetransmission signal processing unit 356, according to the instructionsfrom the state change control unit 358. Besides, the power control unit360 executes transmission power control and electrical power control(for example, control of power ON/OFF) of the RF unit 352, according tothe instructions from the state change control unit 358.

FIG. 4 is an explanatory drawing about state change of the base station2 as the second base station. The base station 2 has two operationstates as shown in FIG. 4. The first operation state is an active stateSt_11 in which the base station 2 can form the radio link 1200 with themobile station 100 which resides in the cell 12. The second operationstate is a radio transmission suspension state St_12. In the radiotransmission suspension state St_12, radio signal transmitted from thebase station 2 to the mobile station 100 is suspended and radiocommunication between the base station 2 and the mobile station 100 inthe cell 12 becomes impossible. The base station 2 executes the changefrom a certain operation state to another operation state wheneverconditions like FIG. 4 are satisfied. A condition on which the basestation 2 changes from the active state St_11 to the radio transmissionsuspension state St_12 is, for example, a condition that communicationof the mobile station 100 in the base station 2 is disconnected and amobile station which connects with the base station 2 does not exist anymore. For example, when there exists a start request (that is, requestfor change from the radio transmission suspension state St_12 to theactive state St_11) from the RNC 200 to the base station 2, theoperation state of the base station 2 changes from the radiotransmission suspension state St_12 to the active state St_11. Further,the “radio transmission suspension state” means, specifically, forexample, a state in which electrical power or transmission function ofthe transmission signal processing unit 356 or the RF unit 352 areturned off by the power control unit 360, and transmission from the basestation 2 to the mobile station 100 is suspended.

FIG. 5 is a sequence chart showing an example of operation of the radiocommunications system, when the base station 2 changes from the activestate St_11 to the radio transmission suspension state St_12. FIG. 5explains operation sequence among the mobile station 100, the basestation 2 and the RNC 200.

The base station 2 is in communication with the mobile station 100 inthe cell 12 (Step S1). Here, in general, a mobile station measuresreceived power in the mobile station of the pilot signal of the basestation (for example, the base station 1 and the base station 2 inFIG. 1) which is registered within a measurement cell set which isinformation reported from the base station in communication, and reportthe measurement result concerned to the RNC 200 periodically via thebase station in communication. Here, the measurement cell set is a setwhich includes cells (base stations) which become a target for which themobile station measures received power of the pilot signal transmittedfrom the base station. The measurement cell set is prepared for in eachbase station. The measurement cell set is reported to the mobile stationunder the communication. In general, own base station and the basestations around it are registered in the measurement cell set.

Here, by a certain reason, the mobile station 100 carries outcommunication disconnect processing to the base station 2 (Step S2). Thebase station 2 which received the communication disconnect request fromthe mobile station 100 confirms whether a mobile station incommunication exists other than the mobile station 100 in the cell 12 ofown base station (Step S3).

In case it is confirmed that a mobile station in communication does notexist other than the mobile station 100, the base station 2 transmits areceived power status confirmation request to the RNC 200 (Step S4).Here, the received power status confirmation request issued from thebase station 2 is described. The RNC 200 has a result which comparedreceived power information of the pilot signal of the base station 2 ina mobile station (for example, the mobile station 101 in communicationwith the base station 1 in FIG. 1) in communication with other basestation in the neighborhood of the cell 12 with a predeterminedthreshold value. The received power status confirmation request meansprocessing in which the base station 2 requests this comparison resultto the RNC 200. The RNC 200 which received the request concernedcompares a measurement result of a mobile station (here, the mobilestation 101) which received the pilot signal of the base station 2 andperformed measurement with the threshold value (Step S5). Next, the RNC200 transmits the comparison result (received power status in the mobilestation) to the base station 2 (Step S6).

In case the comparison result mentioned above goes below the thresholdvalue, and fixed time (5 seconds, for example) has passed, the basestation 2 lowers transmission power of a transmission signal includingthe pilot signal gradually (for example, 1 dB per 0.1 second) (Step S7).Further, while the base station 2 is performing lowering processing oftransmission power, the RNC 200 keeps comparing the measurement resultof the received power of the pilot signal in the mobile station with thethreshold value and keeps transmitting the comparison result to the basestation 2. However, when a state change report of Step S11 describedbelow is received, transmission of the comparison result is suspended.Next, while the base station 2 is lowering transmission power, whether aconnection request from a new mobile station exists is confirmed (StepS8). Also, at the same time, whether the received power in the mobilestation transmitted from the RNC 200 does not exceed the threshold valueis confirmed (Step S9). In case there are no new connection requests andthe received power concerned is lower than the threshold value, the basestation 2 performs processing from Step S5 to Step S9 repeatedly untiltransmission power goes down for fixed quantity (20 dB, for example)(that is, until transmission power becomes 1/100 of power in the activestate St_11).

In case transmission power of the base station 2 goes below thepredetermined threshold value (Step S10), the base station 2 notifiesthe RNC 200 of change to the radio transmission suspension state St_12(Step S11). And the base station 2 which transmitted a state changereport suspends radio transmission to the mobile station and changes tothe radio transmission suspension state St_12 (Step S12). The RNC 200which received the report concerned transmits a notification to theeffect that the change report of the base station 2 was received by theRNC 200 to the base station 2 (Step S13). Also, together withtransmitting the reception notification concerned, the RNC 200 directsthe base station 1 to delete the base station 2 from the measurementcell set (Step 14). And the base station 1 which received the directionconcerned updates the measurement cell set of the base station 1 anddeletes the base station 2 (Step S15).

An example of operation of the base station 2 in case of change from theactive state St_11 to the radio transmission suspension state St_12 isdescribed using a flow chart of FIG. 6 and a base station block diagramof FIG. 3. Further, processing described in the flow concerned (that is,processing of change from the active state St_11 to the radiotransmission suspension state St_12) is carried out when either offollowing three conditions is satisfied. The first condition (1) is,when communication of the mobile station 100 in communication in thecell 12 ended (in other words, in case a mobile station whichcommunicates with the base station 2 does not exist any more). Thesecond condition (2) is, when received power of the pilot signal of thebase station 2 in the mobile station 100 which is in communication withother base station goes below a threshold value and fixed time haspassed. The third condition (3) is, when starting of communication afterchange from the radio transmission suspension state St_12 to the activestate St_11 cannot be performed, which is described below.

When either of the conditions (1)-(3) mentioned above is satisfied, theload management unit 364 of the base station 2 judges, as load data ofthe base station 2, a presence of a mobile station in communication(whether number of a mobile station is 0, or whether communicationtraffic is 0) (Step S20). In case a mobile station in communication withthe base station 2 is judged “to exist” (in case judged as No in StepS20), the load management unit 364 issues a direction to the statechange control unit 358 to keep an operation state of the base station 2in the active state St_11 (Step S29). As a result, the state changecontrol unit 358 directs the power control unit 360 to keep transmissionpower in a usual operation state, and as a result, the active statewhich is a usual communication state is kept.

In case a mobile station in communication is judged “not to exist” (incase judged as Yes in Step S20), the load management unit 364 inquiresof the RNC 200 via the network communication unit 350 a received powerstatus (whether the received power concerned exceeds a threshold value)of the pilot signal of the base station 2 in a mobile station (themobile station 101 in FIG. 1, for example) which exists around the cell12 (Step S21). In case the received power concerned exceeds thethreshold value (in case judged as Yes in Step S21), because the networkcommunication unit 350 receives from the RNC 200 a notice to the effectthat received power of the pilot signal of the base station 2 in themobile station exceeds the threshold value, it outputs the notificationto the state change control unit 358. As a result, the state changecontrol unit 358 issues a direction to keep an operation state of basestation 2 in the active state St_11 (Step S29). On the other hand, incase the received power concerned is lower than the threshold value (incase judged as No in Step S21), because the network communication unit350 receives from the RNC 200 a notice to the effect that received powerof the pilot signal of the base station 2 in the mobile station does notexceed the threshold value, it outputs the notification to the statechange control unit 358. As a result, the state change control unit 358issues a direction to the power control unit 360 to lower graduallytransmission power of a control signal including the pilot signal. Thepower control unit 360 which received the direction to lowertransmission power outputs a direction to lower gradually transmissionpower of the transmission signal processing unit 356 (Step S22).

While the transmission signal processing unit 356 is loweringtransmission power by control of the power control unit 360, the basestation 2 carries out processing described in Step S23 and Step S24repeatedly. The reception signal processing unit 354 confirms whether anew connection request from the mobile station in the cell 12 of ownbase station exists (Step S23). The network communication unit 350receives from the RNC 200 periodically received power status of the basestation 2 in the mobile station, and confirms it (Step S24). In case anew connection request exists from the mobile station in the cell 12while lowering transmission power (in case judged as Yes in Step S23) ora notification is received from the RNC 200 to the effect that receivedpower of the pilot signal of the base station 2 in the mobile stationwhich is in communication with other base station (the mobile station101 in FIG. 1, for example) exceeds a threshold value (in case judged asYes in Step S24), at least one of the reception signal processing unit354 and the network communication unit 350 outputs to the state changecontrol unit 358 control information or a direction to make transmissionpower rise to a prescribed value. By this control information or thedirection, the state change control unit 358 issues a direction to thepower control unit 360 to make transmission power of a control signalincluding the pilot signal in the transmission signal processing unit356 rise to a prescribed value. As a result, the power control unit 360controls the transmission signal processing unit 356, makes transmissionpower rise (Step S28), and keeps an operation state of the base station2 in the active state St_11 (Step S29).

On the other hand, in case no new connection requests exist from themobile station in the cell 12 while lowering transmission power (in casejudged as No in Step S23) and the base station 2 does not receive anotification from the RNC 200 to the effect that a received power of thepilot signal of the base station 2 in the mobile station which is incommunication with other base station exceeds a threshold value (in casejudged as No in Step S24), the transmission signal processing unit 356judges whether transmission power is lowered to a predeterminedthreshold value (Step S25). At the time when being lowered to thethreshold value, the transmission signal processing unit 356 notifiesthe state change control unit 358 and further, the state change controlunit 358 reports to the RNC 200 via the network communication unit 350to the effect that the base station 2 changes to the radio transmissionsuspension state St_12 (Step S26). After reporting state change to theRNC 200, the state change control unit 358 issues a direction to thepower control unit 360 to suspend transmission of the pilot signal ofthe transmission signal processing unit 356 (Step S27). As a result, anoperation state of the base station 2 becomes the radio transmissionsuspension state St_12.

Further, transmission power lowering processing of the base station 2 isnot limited to the above. For example, the power control unit 360 or thetransmission signal processing unit 356 of the base station 2 can lowertransmission power to a predetermined value not gradually but quickly.In that case, at least processing of Step S23, and depending on thecase, processing of Step S24 in FIG. 6 can also be omitted. Here,“predetermined value” mentioned above includes a state when a signal isnot outputted at all, that is, power “0” (“0” watt, for example). FIG. 7is a sequence chart showing an example of operation, of the radiocommunications system in the first exemplary embodiment on the occasionwhen the second base station changes from a radio transmissionsuspension state to an active state and hands over a mobile stationwhich are accommodated in the first base station to the second basestation. Here, the mobile station 100 is supposed to communicate firstwith the base station 1 newly.

In the neighborhood of the base station 2 which is in the radiotransmission suspension state St_12, the mobile station 100 transmits aconnection request concerning an outgoing call and so on to the basestation 1 in the neighborhood of the base station 2 (Step S30). The basestation 1 which received the connection request of the mobile station100 notifies the RNC 200 that the mobile station 100 issued theconnection request (Step S31). The base station 1 measures load based ontraffic amount and so on of own base station (Step S32). The basestation 1 transmits measured load data to the RNC 200. (Step S33). TheRNC 200 compares the load data of the base station 1 with apredetermined threshold value (Step S34).

In case the load data exceeds the threshold value (in other words, incase load of the base station 1 is heavy), the RNC 200 transmits a startrequest to the base station 2 (Step S35). The base station 2 whichreceived the start request from the RNC 200 makes an operation statechange from the radio transmission suspension state St_12 to the activestate St_11 (Step S36). The base station 2 which changed to the activestate St_11 starts transmission of a control signal including the pilotsignal with prescribed transmission power (Step S37). The base station 2which started transmission of the pilot signal reports (start report) tothe RNC 200 that it changed to active (Step S38).

The RNC 200 which received the start report from the base station 2issues a connection permission notification to the base station 1 to theeffect that the base station 1 and the mobile station 100 are permittedto connect (Step S39). The base station 1 which received the connectionpermission notification from the RNC 200 issues the connectionpermission notification to the mobile station 100 (Step S40). The mobilestation 100 which obtained the connection permission from the basestation 1 starts communication of user data (main information ofcommunication (sound or data, for example)) with the base station 1(Step S41).

Also, the RNC 200 directs to add the cell 12 of the base station 2 tothe measurement cell set of the base station 1 concurrently with StepS39 (Step S42). The base station 1 which received from the RNC 200 theaddition direction to the measurement cell set adds the base station 2to the measurement cell set (Step S43) and notifies the mobile stationwhich is in communication of update of the measurement cell set (StepS44).

The mobile station 100 which received from the base station 1 the updatenotification of the measurement cell set measures received power of thepilot signal which is transmitted from the base stations (the basestation 1 and the base station 2 in FIG. 1, for example) registeredwithin the new measurement cell set (Step S45). And the measurementresult is transmitted to the base station 1 (Step S46). The base station1 transmits the measurement result received from the mobile station 100to the RNC 200 (Step S47). The RNC 200 which received the measurementresult compares received power of the base station 2 in the mobilestation 100 with a threshold value (Step S48). At that time, when thereceived power of the pilot signal of the base station 2 is higher thanthe threshold value, the RNC 200 notifies the base station 1 and thebase station 2 respectively, of a command to the effect that the mobilestation 100 is handed over from the base station 1 to the base station 2(Step S49 and Step S50).

The base station 1 which received the handover command concerned fromthe RNC 200 issues to the mobile station 100 a command to the effectthat communication is handed over to the base station 2 (Step S51). Thebase station 2 hands over communication of the mobile station 100 fromthe base station 1, and starts user data communication with the mobilestation 100 (Step S52). The base station 2 reports to the RNC 200 to theeffect that a connection is established with the mobile station 100(Step S53). The RNC 200 notifies the base station 2 to the effect thatthe connection establishment is confirmed (Step S54). The RNC 200commands the base station 1 to disconnect communication with the mobilestation 100 (Step S55). The base station 1 which received thecommunication disconnect command disconnects communication with themobile station 100 (Step S56).

By the processing mentioned above, handover processing of the mobilestation 100 from the base station 1 to the base station 2 ends.

Further, in Step S48, in case received power of the base station 2 islower than the threshold value, the RNC 200 commands the base station 2to call a flow (shown in FIG. 6) which changes to the radio transmissionsuspension state. St_12. The base station 2 which received the commandconcerned operates based on the calling condition (3) of a flow tochange from the active state St_11 to the radio transmission suspensionstate St_12 mentioned above.

FIG. 8 is a flow chart showing an example of operation of the secondbase station in the first exemplary embodiment on the occasion when thesecond base station changes from a radio transmission suspension stateto an active state and hands over a mobile station which areaccommodated the first base station to the second base station.

The base station 2 in the radio transmission suspension state St_12starts change from the radio transmission suspension state St_12 to theactive state St_11 by receiving a start request from the RNC 200 afterthe mobile station 100 transmitted a connection request to the basestation 1.

The network communication unit 350 of the base station 2 which receivedthe start request from the RNC 200 outputs the start request to thestate change control unit 358. As a result, the state change controlunit 358 issues a direction to make an operation state of the basestation 2 change from the radio transmission suspension state St_12 tothe active state St_11. By the direction of the state change controlunit 358, the power control unit 360 issues a direction to maketransmission power of a control signal including the pilot signal in thetransmission signal processing unit 356 increase to a prescribed value.As a result, transmission of the pilot signal from the base station 2starts (Step S70). After starting transmission of the pilot signal, thetransmission signal processing unit 356 of the base station 2 reports(start report) to the RNC 200 via the network communication unit 300that it changed to the active state (Step 71).

Next, the transmission signal processing unit 356 of the base station 2confirms with the network communication unit 350 whether a handovercommand is received from the RNC 200 (Step S72).

When the handover command is received (in case of Yes in Step S72), thetransmission signal processing unit 356 and the reception signalprocessing unit 354 establish a connection with the mobile station 100and start user data communication (Step S73). The transmission signalprocessing unit 356 reports to the RNC 200 via the network communicationunit 350 to the effect that the connection between the base station 2and the mobile station 100 is established (Step S74). By reporting tothe RNC 200 that the base station 2 and the mobile station 100 havestarted user data communication, processing shown in this flow (that is,processing to make change from the radio transmission suspension stateSt_12 to the active state St_11, and to handover a mobile station whichthe base station 1 accommodates to the base station 2) ends.

Further, in case there are no handover commands (in case judged as No inStep S72), the state change control unit of the base station 2 judgesthat the base station 2 and the mobile station 100 cannot communicate orenough quality cannot be secured even if they can communicate.Accordingly, because the meaning is lost to keep the active state St_11concerning communication with this mobile station 100, the base station2 carries out processing to return the operation state of the basestation 2 from the active state St_11 to the radio transmissionsuspension state St_12 (Step S75). Because this change processing isalready described using FIG. 6, its description will be omitted.

In the radio communications system according to the first exemplaryembodiment described above, the base station 2 is characterized by, inthe radio transmission suspension state, when communication between thebase station 1 and the mobile station 100 is started (specifically, whenthe mobile station 100 performs a connection request to the base station1) and a predetermined condition is satisfied, starting transmission ofthe pilot signal with predetermined power.

That is, when communication is started between the mobile station 100and the base station 1 as the first base station (that is, a mobilestation having possibility to be a handover target exists) and apredetermined condition is satisfied (in case load of the base station 1is heavy, for example), the base station 2 as the second base station,for the first time at that point, starts transmission of the pilotsignal to the mobile station 100 (that is, it changes from the radiotransmission suspension state St_12 to the active state St_11).

Specifically, the base station 2 includes: a means (the networkcommunication unit 350, for example) which receives a start requesttransmitted from other equipment when communication between the basestation 1 and the mobile station 100 is started and a predeterminedcondition is satisfied, and a means (the state change control unit 358and the power control unit 360, for example) which starts transmissionof the pilot signal with predetermined power based on the start request.

Thus, in case of the radio communications system of the first exemplaryembodiment, only in case a mobile station with high possibility ofhandover from an adjacent base station to own base station exists forcertain, own base station can be set to the active state St_11.Accordingly, for example, returning to an active state wastefully likeJapanese Patent Application Laid-Open No. 2003-37555 is avoided, and asa result, it becomes possible, with more certainty, to suppressconsumption of electric power of the base station and avoid radiointerference between the base stations.

Further, the base station 2 of the radio communications system accordingto the first exemplary embodiment judges, in case there are no handovercommands from the RNC 200 after changing to the active state St_11, thata mobile station which communicates with the base station 2 does notexist, the base station 2 and the mobile station 100 cannot communicate,or enough quality cannot be secured even if they can communicate. And inthat case, the base station 2 judges that the meaning to keep the activestate St_11 concerning communication with this mobile station 100 islost and carries out processing to return the operation state of thebase station 2 from the active state St_11 to the radio transmissionsuspension state St_12.

That is, the base station 2 includes a means to suspend transmission ofthe pilot signal (the state change control unit 358 and the powercontrol unit 360, for example) after it has started transmission of thepilot signal, in case a mobile station which communicates with the basestation 2 does not exist, the base station 2 and a mobile station cannotcommunicate, or enough quality cannot be secured even if they cancommunicate.

By performing more careful state change control as above, it becomespossible, with more certainty, to suppress consumption of electric powerof the base station and avoid radio interference between the basestations.

Further, in the description mentioned above, a case for one second basestation (that is, a base station which changes between the radiotransmission suspension state St_12 and the active state St_11appropriately, specifically the base station 2 in FIG. 1) is given as anexample. However, number of second base stations may be plural. When acase is assumed where second base stations exist in plural, according tocontrol described in Japanese Patent Application Laid-Open No.2003-37555, all second base stations return to the active state St_11and those other than the base station which is in communication with amobile station keep the active state wastefully. In case of the presentexemplary embodiment, there is also a possibility that all second basestations may be in the active state St_11 once. However, because asecond base station which did not receive a handover command from otherequipment (the RNC 200, for example) changes to the radio transmissionsuspension state St_12 immediately, different from Japanese PatentApplication Laid-Open No. 2003-37555, improvement effect of powerconsumption reduction and avoidance of radio interference can beobtained.

Further, in the first exemplary embodiment mentioned above, although“communication traffic” of the base station 1 was given as an example ofthe load data of the base station 1, the load data is not limited tocommunication traffic. The load data can, for example, be made as“number of mobile stations in connection” of the base station 1. It isalso possible to adopt both “communication traffic information” and“information of number of mobile stations in connection” as load data.

Also, in the first exemplary embodiment mentioned above, sequence ofprocessing (specifically, processing of Steps S39-S41 in FIG. 7) inwhich the mobile station 100 which obtained connection permission fromthe base station 1 (the first base station) starts communication of userdata with the base station 1 is not limited to the sequence shown inFIG. 7. For example, the user data communication start processingconcerned may be performed before processing in which the base station 2(the second base station) changes to the active state St_11 (Step S36)or may be performed after processing in which the base station 1 addsthe base station 2 to the measurement cell set (Step S43). As an exampleof a case to perform starting of the user data communication concernedbefore processing in which the base station 2 changes to the activestate St_11, for example, a case to perform starting of the user datacommunication concerned before load measurement of the base station 1(Step S32) is performed can be given as an example. In this case,because communication of user data has already been started, the basestation 1 can perform more correct load measurement. Or, starting of theuser data communication concerned may be performed after processing totransmit a start request from the RNC 200 to the base station 2 (StepS35). As a result, it becomes possible to reduce connection delay. Onthe other hand, as an example of a case to perform starting of the userdata communication concerned after processing in which the base station1 adds the base station 2 to the measurement cell set, for example, acase to perform it after update notification processing (Step S44) ofthe measurement cell set from the base station 1 to the mobile station(for example, the mobile station 101 in FIG. 1) in communication withthe base station 1 concerned can be given as an example.

Second Exemplary Embodiment

Hereinafter, a radio communications system according to the secondexemplary embodiment of the present invention is described. The entirestructure of this radio communications system is identical with theradio communications system of the first exemplary embodiment shown inFIG. 1. However, in case of the second exemplary embodiment, it issupposed that, in FIG. 1, the base station 1 as the first base stationis replaced by the base station 4 described below. The difference of thesecond exemplary embodiment from the first exemplary embodiment existsin a structure of the first base station. Hereinafter, a base station asthe first base station in the radio communications system of the secondexemplary embodiment is newly referred to as a base station 4.Accordingly, a base station corresponding to the second base station inthis radio communications system remains as the base station 2 of thefirst exemplary embodiment (refer to FIG. 3).

FIG. 9 is a block diagram showing an example of the base station 4 asthe first base station of the second exemplary embodiment. The basestation 4 further includes a location information acquisition unit 450in addition to a structure included in the base station 1 as the firstbase station in the first exemplary embodiment shown in FIG. 2. Becausea structure and an operation of other components (that is, a networkcommunication unit 300, a RF unit 302, a reception signal processingunit 304, a transmission signal processing unit 306, a load managementunit 308 and an antenna 310) are the same as the base station 1, theirdescription will be omitted. The location information acquisition unit450 acquires or detects location of a mobile station residing in thecell 11 of the base station 4 based on information obtained from thereception signal processing unit 304.

Further, in the radio communications system of this second exemplaryembodiment and a radio communications system of the third exemplaryembodiment described thereafter, operation sequence of the radiocommunications system in case the second base station changes from theactive state to the radio transmission suspension state is the same asoperation sequence of the radio communications system in the firstexemplary embodiment (refer to FIG. 5). Also, in the radiocommunications system of this second exemplary embodiment and the radiocommunications system of the third exemplary embodiment describedthereafter, an operation flow of the second base station in case ofchange from the active state to the radio transmission suspension stateis the same as the operation flow of the second base station (refer toFIG. 6). Accordingly, hereinafter, description about the operationsequence mentioned above and the operation flow mentioned above will beomitted. Also, in the radio communications system of this secondexemplary embodiment and a radio communications system of the thirdexemplary embodiment described thereafter, because an operation flow incase the second base station changes from the radio transmissionsuspension state St_12 to the active state St_11 is also the same as anoperation flow of the second base station in the first exemplaryembodiment (refer to FIG. 8), hereinafter, its description will also beomitted.

FIG. 10 is a sequence chart showing an example of operation of the radiocommunications system in the second exemplary embodiment on the occasionwhen the second base station changes from a radio transmissionsuspension state to an active state and hands over a mobile stationwhich are accommodated in the first base station to the second basestation. In case of this exemplary embodiment, as a condition to changestate of the base station 2 from the radio transmission suspension stateSt_12 to the active state St_11, load data between the mobile station100 and the base station 4 and location information of the mobilestation are used.

In the neighborhood of the base station 2 which is in the radiotransmission suspension state St_12, the mobile station 100 (refer toFIG. 1) transmits a connection request to the base station 4 (thoughdescribed as the base station 1 in FIG. 1, hereinafter, considered asthe base station 4 as mentioned above) in the neighborhood of the basestation 2 (Step S149). The base station 4 which received the connectionrequest of the mobile station 100 notifies the RNC 200 that the mobilestation 100 issued the connection request (Step S150). The base station4 measures load data of own base station (Step S151). The base station 1transmits measured load data to the RNC 200 (Step S152). The RNC 200compares the load data of the base station 4 with a predeterminedthreshold value (Step S153). Also, after transmitting the load data, thebase station 4 requests location information of the mobile station 100to the mobile station 100 (Step S154). The mobile station 100 whichreceived the location information request from the base station 4acquires location information using a location information acquisitionmeans (GPS (Global Positioning System), for example) (Step S155). Themobile station 100 which acquired the location information transmits thelocation information to the base station 4 (Step S156). The base station4 which acquired the location information of the mobile stationtransmits the location information of the mobile station 100 to the RNC200 (Step S157).

Next, processing in case load data of the base station 4 is judged inStep S153 to be higher than a threshold value is described. In thiscase, the RNC 200 compares, using location information of the mobilestation 100, location relationship between the mobile station 100 and anarea possible to communicate with the base station 2 (the cell 12 inFIG. 1) which is at present in the radio transmission suspension stateSt_12 and, when it becomes the active state St_11, is to be registeredwithin the measurement cell set of the base station 4 (Step S158). As aresult of location relationship comparison, in case the base station 2which can communicate with the mobile station 100 (a mobile stationwhich transmitted a connection request to the base station 4 in theneighborhood of the base station 2) is found, the RNC 200 determinesthis base station 2 as a base station to start (Step S159). Accordingly,the RNC 200 transmits a start request to the base station 2 (Step S160).The base station 2 which received the start request from the RNC 200makes an operation state change from the radio transmission suspensionstate St_12 to the active state St_11 (Step S161). The base station 2which changed to the active state St_11 starts transmission of a controlsignal including the pilot signal with prescribed power (Step S162). Thebase station 2 which changed to the active state reports (start report)to the RNC 200 that it changed to the active state (Step S163). The RNC200 which received information to the effect that the base station 2changed to the active state issues a connection permission notificationto the base station 4 to the effect that the base station 4 and themobile station 100 are permitted to connect (Step S164). The basestation 4 which received the connection permission notification from theRNC 200 issues the connection permission notification to the mobilestation 100 (Step S165). As a result, user data communication is startedbetween the base station 4 and the mobile station 100 (Step S166).

After transmitting user data communication permission with the mobilestation 100 to base station 4, the RNC 200 directs the base station 4 toadd the cell 12 of the base station 2 to a measurement cell set of thebase station 4 (Step S167). The base station 4 which received thedirection adds the cell 12 to the measurement cell set (Step S168) andnotifies the mobile station which is in communication of update of themeasurement cell set (Step S169).

The mobile station 100 which received the update notification of themeasurement cell set from the base station 4 measures the pilot signalreceived power which is transmitted from the base stations (for example,the base station 4 and the base station 2 in FIG. 1) registered withinthe new measurement cell set (Step 170). The measurement result istransmitted to the base station 4 (Step S171). The base station 4transmits the measurement result received from the mobile station 100 tothe RNC 200 (Step S172). The RNC 200 which received the measurementresult compares received power of the base station 2 in the mobilestation 100 with the threshold value (Step S173). In case the receivedpower of the base station 2 is higher than the threshold value, the RNC200 notifies the base station 4 and the base station 2 respectively, ofa command to the effect that the mobile station 100 is handed over fromthe base station 4 to the base station 2 (Step S174 and Step S175).

The base station 4 which received the handover command concerned fromthe RNC 200 issues to the mobile station 100 a command to the effectthat communication is handed over to the base station 2 (Step S176). Thebase station 2 hands over communication of the mobile station 100 fromthe base station 4, and starts user data communication with the mobilestation 100 (Step S177). The base station 2 reports on the RNC 200 aboutconnection establishment with the mobile station 100 (Step S178).

The RNC200 notifies the base station 2 a confirmation of connectionestablishment based on this report (step S179). The RNC 200 commands thebase station 4 to disconnect communication with the mobile station 100(Step S180). The base station 4 which received communication disconnectcommand disconnects communication with the mobile station 100 (StepS181).

By the processing mentioned above, handover processing of the mobilestation 100 from the base station 4 to the base station 2 ends.

As described above, the second exemplary embodiment can, by using loaddata of the base station 4 and location information of the mobilestation, select the base station to start (that is, to change from theradio transmission suspension state St_12 to the active state St_11)without waste and efficiently. That is, when load of the base station 4as the first base station is light, the base station 2 as the secondbase station can keep the radio transmission suspension state St_12, andalso by using location information of the mobile station, not onlyleveling of traffic between the base stations is achieved but also radiointerference to other communication and wasteful power consumption aresuppressed. In this case, a base station which accommodates a lot ofmobile stations can be started by priority around the base station inthe radio transmission suspension state regardless of order of aconnection request or connection of a mobile station.

Here, in FIG. 1, the mobile station 100 resides in the cell 12 which iswithin a range possible to communicate with the base station 2.Accordingly, by starting the base station 2, the mobile station 100 canbe accommodated by the base station 2. However, the mobile station 101is located in a place where the base station 2 can not be used.Accordingly, even if the mobile station 101 enters newly cell 11 of thebase station 4 and requests connection, and load of the base station 4increases as a result, it is not possible to handover to the basestation 2 and to accommodate the mobile station 101. That is, load ofthe base station 4 can not be reduced. This exemplary embodiment mayresolve this problem as follows.

That is, when load of the base station 4 exceeds a threshold value by aconnection request from the mobile station 101, the RNC 200 detectslocation of all mobile stations which are in connection with the basestation 4 and searches out a mobile station that exists in the cell 12of the base station 2 from among those mobile stations. In case nosmaller than one mobile station which is in connection with the basestation 4 and resides in the cell 12 exists, the RNC 200 starts the basestation 2, and hands over at least one mobile station which is inconnection with the base station 4 and resides in the cell 12 from thebase station 4 to the base station 2. Thus, load of the base station 4can be reduced.

Also, concerning acquisition and detection of location information of amobile station, for example, information of GPS (Global PositioningSystem), information of AGPS (Assisted GPS) which measures location byGPS and information of a base station or information of AFLT (AdvancedForward Link Trilateration) and so on can be used.

Also, as another modification example of the second exemplaryembodiment, description is made with reference to FIG. 11. In FIG. 11, abase station 2-1 and a base station 2-2 as the second base station areat present in the radio transmission suspension state St_12. And forexample, suppose a case where three mobile stations 104-106 located in acell 15 of the base station 2-1 issue a connection request to the basestation 4 as the first base station and one mobile station 107 locatedin a cell 16 of the base station 2-2 issues a connection request to thebase station 4.

In case load of the base station 4 exceeds a threshold value by theseconnection requests, the RNC 200 detects location of all mobile stationswhich are in connection with the base station 4 and finds a mobilestation among those mobile stations which resides in the cell 15 of thebase station 2-1 and in the cell 16 of the base station 2-2. In thiscase, as mentioned above, three mobile stations 104-106 reside in thecell 15 of the base station 2-1, and one mobile station 107 resides inthe cell 16 of the base station 2-2. In this case, it is possible forthe RNC 200, for example, in selection of base station to start in StepS159 of FIG. 10, to select, out of the base station 2-1 and the basestation 2-2 which are in the radio transmission suspension state St_12,the base station 2-1 which has more residing mobile stations, totransmit a start request of Step S161 to the base station 2-1, and tostart it with priority.

Also, in the second exemplary embodiment mentioned above, sequence ofprocessing (specifically, processing of Steps S164-S166 in FIG. 10) inwhich the mobile station 100 which obtained connection permission fromthe base station 4 (first base station) starts communication of userdata with the base station 4 is not limited to the sequence shown inFIG. 10. For example, the user data communication start processingconcerned may be performed before processing in which the base station 2(second base station) changes to the active state St_11 (Step S161) ormay be performed after processing in which the base station 4 adds thebase station 2 to the measurement cell set (Step S168). As an example ofa case to perform starting of the user data communication concernedbefore processing in which the base station 2 changes to the activestate St_11, for example, a case to perform starting of the user datacommunication concerned before load measurement of the base station 4(Step S151) is performed can be given as an example. In this case,because communication of user data has already been started, the basestation 4 can perform more correct load measurement. Or, starting of theuser data communication concerned may be performed after processing totransmit a start request from the RNC 200 to the base station 2 (StepS160). As a result, it becomes possible to reduce connection delay. Onthe other hand, as an example of a case to perform starting of the userdata communication concerned after processing in which the base station4 adds the base station 2 to the measurement cell set, for example, acase to perform it after update notification processing (Step S169) ofthe measurement cell set from the base station 4 to a mobile station(for example, the mobile station 101 in FIG. 1) in communication withthe base station 4 concerned can be given as an example.

Third Exemplary Embodiment

FIG. 12 is a block diagram showing an example of a radio communicationssystem according to the third exemplary embodiment of the presentinvention. The radio communications system installs the base station 2as the second base station which performs a radio transmissionsuspension, in the cell 11 in which a base station 5 as the first basestation can communicate, as an adjacent base station of the base station5. Here, a cell in which base station 2 can communicate is a cell 17. Inthis case, at least part of the cell 17 and the cell 11 overlap.

Further, in this radio communications system, the base station 5 as thefirst base station has the identical structure with the base station 4of the second exemplary embodiment, and also the base station 2 as thesecond base station has the identical structure with the base station 2of the first and second exemplary embodiment. Accordingly, descriptionwill be made hereafter, for the base station 5, by using a configurationof the base station 4 of FIG. 9, while for the base station 2, by usinga configuration of the base station 2 shown in FIG. 3.

Also, the RNC 200 includes a relation table (not shown) which relatesunique ID of a mobile station and a start target base station. When aconnection request is received from a mobile station in the cell 11, theRNC 200 refers to the relation table and sets an adjacent base station(for example, the base station 2 in FIG. 12) related to the unique ID ofthe mobile station concerned as a base station of start target. And themobile station concerned performs user data communication with therelated adjacent base station.

FIG. 13 is a sequence chart showing an example of operation of the radiocommunications system in the third exemplary embodiment on the occasionwhen the second base station changes from a radio transmissionsuspension state to an active state and hands over a mobile stationwhich are accommodated in the first base station to the second basestation.

In the neighborhood of the base station 2 which is in the radiotransmission suspension state St_12, a mobile station 108 transmits aconnection request to the base station 5 in the neighborhood of the basestation 2 (Step S200). The base station 5 which received the connectionrequest of the mobile station 108 notifies the RNC 200 that the mobilestation 108 issued the connection request (Step S201).

The RNC 200 receives the connection request of the mobile station 108from the base station 5 and extracts unique ID of the mobile station 108from the received connection request. By referring to a relation tableand a table of all base stations registered within a measurement cellset of the base station 5, the RNC 200 confirms a presence of a basestation which is related to the extracted unique ID and is in the radiotransmission suspension state (Step S202). On the other hand, the basestation 5 requests location information of the mobile station 108 to themobile station 108 (Step S203). The mobile station 108 which receivedthe request for location information, acquires location information byusing a location information acquisition means (Step S204). The mobilestation 108 which acquired the location information transmits thelocation information to the base station 5 (Step S205). The base station5 which acquired the location information of the mobile station 108transmits the location information of the mobile station 108 to the RNC200 (Step S206).

Next, when a base station to which unique ID of the mobile station 108is related exists in base stations registered within a measurement cellset of the base station 5 in Step S202, locations of the related basestation and the mobile station are compared (Step S207). And the RNC 200determines whether the base station is started based on unique ID of themobile station and location information of the base station related tothe ID (Step S208). Specifically, the RNC 200 judges whether the mobilestation 108 resides in the cell 17 of a base station (in this case, thebase station 2) which is related to unique ID of the mobile station 108and is in a radio transmission suspension state by using locationinformation of the mobile station 108. When the mobile station 108resides in the cell 17 of the base station 2 which is related to theunique ID, the RNC 200 determines to start the base station 2.Accordingly, the RNC 200 transmits a start request to the base station 2(Step S209). The base station 2 which received the start request fromthe RNC 200 changes an operation state from the radio transmissionsuspension state St_12 to the active state St_11 (Step S210). The basestation 2 which changed to the active state St_11 starts transmission ofa control signal including the pilot signal according to prescribedtransmission power (Step S211). The base station 2 which startedtransmission of the pilot signal reports (start report) to the RNC 200that it changed to the active state (Step S212).

The RNC 200 which received the start report from the base station 2issues a connection permission notification which the base station 5 andthe mobile station 108 are permitted to connect, to the base station 5(Step S213). The base station 1 which received the connection permissionnotification from the RNC 200 issues connection permission to the mobilestation 108 (Step S214). The mobile station 108 which obtained theconnection permission from the base station 5 starts user datacommunication with the base station 5 (Step S215).

Also, the RNC 200 directs to add the cell 17 of the base station 2 to ameasurement cell set of the base station 5 concurrently with Step S210(Step S216). The base station 5 which received from the RNC 200 theaddition direction to the measurement cell set adds the base station 2to the measurement cell set (Step S217) and notifies the mobile stationwhich is in communication of update of the measurement cell set (StepS218).

The mobile station 108 which received the update notification of themeasurement cell set from the base station 5 measures received power ofthe pilot signal which is transmitted from base stations (the basestation 5 and the base station 2 in FIG. 12, for example) registeredwithin the new measurement cell set (Step S219). And the measurementresult is transmitted to the base station 5 (Step S220). The basestation 5 transmits the measurement result received from the mobilestation 108 to the RNC 200 (Step S221). The RNC 200 which received themeasurement result compares received power of the base station 2 in themobile station 108 with a threshold value (Step S222). In case thereceived power of the base station 2 is higher than the threshold value,the RNC 200 notifies the base station 5 and the base station 2respectively, of a command to the effect that mobile station 108 ishanded over from the base station 5 to the base station 2 (Step S223 andStep S224).

The base station 5 which received the handover command concerned fromthe RNC 200 issues to the mobile station 108 a command to the effectthat communication is handed over to the base station 2 (Step S225). Thebase station 2 hands over communication of mobile station 108 from basestation 5, and starts user data communication with mobile station 108(Step S226). The base station 2 reports on a notification that acommunication is established between the mobile station 108 to theRNC200 (step S227).

By this report, the RNC 200 sends a notification that the connectionestablishment is confirmed, to the base station 2 (Step S228). Further,the RNC 200 commands the base station 5 to disconnect communication withthe mobile station 108 (Step S229). The base station 5 which receivedthe communication disconnect command disconnects communication with themobile station 108 (Step S230).

By the processing mentioned above, handover processing of the mobilestation 108 from the base station 5 to the base station 2 ends.

Further, in case a mobile station 109 (refer to FIG. 12) which is notrelated to the base station 2 as the second base station issues acommunication request to the base station 5, when it is found that abase station related to the mobile station 108 does not exist, the RNC200 gives connection permission to the base station 5, and the mobilestation 109 and the base station 5 start to communicate. In other words,the mobile station 109 which is not related to the base station 2communicates with the base station 5 without communicating with the basestation 2.

Further, according to the second and third exemplary embodimentmentioned above, although a case was given as an example where inquiries(for example, in case of the second exemplary embodiment, Step S154 ofFIG. 10) are made from the first base station (for example, in case ofthe second exemplary embodiment, the base station 4) to a mobile stationvoluntarily when location information of a mobile station is acquired,it is not limited to this. The first base station may inquire locationinformation to a mobile station, for example, after receiving adirection from the RNC 200. Also, in case location information can beacquired from a signal of a mobile station which performed a connectionrequest, location information does not need to be inquired newly to themobile station.

Also, in the second and third exemplary embodiment mentioned above,sequence of acquiring load data and location information of the firstbase station (for example, in case of the second exemplary embodiment,the base station 4) is not limited to examples of FIG. 10 (the secondexemplary embodiment) or FIG. 13 (the third exemplary embodiment), andit may be replaced. Also, respective threshold values may be changeddepending on load data and location information. For example, even incase load of the first base station is light, in case a mobile stationexists in the neighborhood of the second base station (for example, incase of the second exemplary embodiment, the base station 2), the secondbase station may be started. On the other hand, even in case load of thefirst base station is heavy, in case the mobile station is within acommunication range of the second base station, the second base stationmay be started.

As described above, by providing the base station related to the mobilestation, the base station which communicates only with the specificmobile station can be set. That is, an emergency call and so on can beconnected with priority compared with other mobile stations.

Further, in the present exemplary embodiment, information used to relatethe mobile station and the base station is not limited to unique ID ofthe mobile station necessarily. For example, calling information (kindsof outgoing calls, for example) can be adopted as information whichrelates the mobile station and the base station. By relating the mobilestation and the base station by calling information, in case a specialoutgoing call such as an emergency call is made, the outgoing callconcerned can be connected with priority compared with other generaloutgoing calls or communication quality of the outgoing call concernedcan be made of high quality compared with other general outgoing calls.

Also, in the third exemplary embodiment mentioned above, although onlythe base station 2 related to unique ID of a mobile station isinstalled, in case another base station 2 which is not related to uniqueID exists, it is possible to operate similar to the third exemplaryembodiment mentioned above.

Also, in the third exemplary embodiment mentioned above, sequence ofprocessing (specifically, processing of Steps S213-S215 in FIG. 13) inwhich the mobile station 108 which obtained connection permission fromthe base station 5 (first base station) starts communication of userdata with the base station 5 is not limited to the sequence shown inFIG. 13. For example, the user data communication start processingconcerned may be performed before processing in which the base station 2(second base station) changes to the active state St_11 (Step S210) ormay be performed after processing in which the base station 5 adds thebase station 2 to the measurement cell set (Step S217). As an example ofa case to perform starting of the user data communication concernedbefore the base station 2 changes to the active state St_11, forexample, a case to perform starting of the user data communicationconcerned before confirmation processing by the RNC 200 about a presenceof a base station which is related to extracted unique ID and is in aradio transmission suspension state (Step S202) can be given as anexample. Or, starting of the user data communication concerned may beperformed after processing which transmits a start request from the RNC200 to the base station 2 (Step S209). In this case, it becomes possibleto reduce connection delay. On the other hand, as an example of a caseto perform starting of the user data communication concerned afterprocessing in which the base station 5 adds the base station 2 to themeasurement cell set, for example, a case to perform it after updatenotification processing (Step S218) of the measurement cell set from thebase station 5 to the mobile station (for example, the mobile station109 in FIG. 12) in communication with the base station 5 concerned canbe given as an example.

OTHER EXAMPLES OF MODIFICATION

In the first to third exemplary embodiments described above, it isdescribed that the handover of the mobile station from the first basestation to the second base station is performed after starting “userdata” communication between the first base station and the mobilestation. However, the handover mentioned above can also be performed ata communication stage of “control signal” in which user data is notcommunicated.

FIG. 14 is a figure describing a first method for carrying out handoverprocessing of a control signal at a communication stage, and a sequencechart showing an example of operation of a radio communications systemon the occasion when the second base station changes from a radiotransmission suspension state to an active state and hands over a mobilestation which are accommodated in the first base station to the secondbase station in the third exemplary embodiment.

Further, in FIG. 14, Step S250 (the mobile station 108 transmits aconnection request to the base station 5)˜Step S262 (the base station 2changes to the active state) are common with Steps S200˜S212 shown inFIG. 13, their description will be omitted.

The RNC 200 which received a start report from the base station 2directs the mobile station 108 via the base station 5 to connect to thebase station 2 (Step S263 and Step S264). The mobile station 108 whichreceived the direction to connect to the base station 2 confirms whetherthe pilot signal of the base station 2 can be received (Step S265). Themobile station 108 which could confirm that the pilot signal could bereceived reports via the base station 5 that the pilot signal of thebase station 2 could be received (Step S266 and Step S267). The RNC 200which confirmed that the mobile station 108 could receive the pilotsignal of the base station 2 transmits to the base station 2 permissionto connect with the mobile station 108 (Step S268). The base station 2which received the connection permission with the mobile station 108transmits connection permission to the mobile station 108 (Step S269).The mobile station 108 which obtained the connection permission from thebase station 5 starts user data communication with the base station 2(Step S270).

Further, in case of starting connection with the base station 2directly, the mobile station 108 which received Step S264 receives thepilot signal of the base station 2, and based on the pilot signal, maytransmit a connection request to the base station 2 newly. At that time,the connection request which the mobile station 108 transmitted to thebase station 5 is discarded after the RNC 200 transmits a connectionbase station direction.

FIG. 15 is a figure describing a second method for carrying out handoverprocessing of a control signal at a communication stage, and a sequencechart showing an example of operation of a radio communications systemon the occasion when the second base station changes from a radiotransmission suspension state to an active state and hands over a mobilestation which are accommodated in the first base station to the secondbase station in the first exemplary embodiment. Further, in order tomake the differences with FIG. 7 clear, in FIG. 15, processing identicalwith processing shown in FIG. 7 is given a step number identical withFIG. 7, and in the following description, a description of a stepidentical with FIG. 7 may be partly omitted.

In the neighborhood of the base station 2 which is in the radiotransmission suspension state St_12, the mobile station 100 transmits aconnection request (that is, “control signal”) which concerns anoutgoing call and so on to the base station 1 in the neighborhoods ofthe base station 2 (Step S30). The base station 1 notifies the RNC 200that the mobile station 100 issued the connection request (Step S31).The base station 1 measures load based on traffic amount and so on ofown base station (Step S32). The base station 1 transmits measured loaddata to the RNC 200 (Step S33). The RNC 200 compares the load data ofthe base station 1 with a predetermined threshold value (Step S34). Incase the load data exceeds the threshold value, the RNC 200 transmits astart request to the base station 2 (Step S35). The base station 2 makesan operation state change from the radio transmission suspension stateSt_12 to the active state St_11 (Step S36). The base station 2 whichchanged to the active state St_11 starts transmission of the controlsignal including the pilot signal with prescribed transmission power(Step S37). The base station 2 which started transmission of the pilotsignal reports (start report) to the RNC 200 that it changed to active(Step S38). Processing of Steps S30-S38 described above is completelythe same as FIG. 7.

Here, in case of FIG. 15, processing required to start user datacommunication (that is, processing shown by Steps S39-S41 in FIG. 7) isomitted. That is, at this stage, only communication of the controlsignal alone is performed and communication of user data has not beenstarted yet.

The RNC 200 which received the start report from the base station 2directs to add the cell 12 of the base station 2 to the measurement cellset of the base station 1 (Step S42). The base station 1 adds the basestation 2 to the measurement cell set (Step S43) and notifies the mobilestation 100 in communication (communication of a control signal only) ofupdate of the measurement cell set (Step S44). The mobile station 100measures received power of the pilot signal which is transmitted fromthe base stations (the base station 1 and the base station 2 in FIG. 1,for example) registered within the new measurement cell set (Step 45).And the measurement result is transmitted to the base station 1 (StepS46). The base station 1 transmits the measurement result received fromthe mobile station 100 to the RNC 200 (Step S47). The RNC 200 whichreceived the measurement result compares received power of the basestation 2 in the mobile station 100 with a threshold value (Step S48).The RNC 200 determines a base station to connect based on the comparisonresult mentioned above (Step S500). For example, in case the receivedpower of the pilot signal of the base station 2 is higher than thethreshold value, the RNC 200 determines the base station 2 as a basestation to connect. In this case, the RNC 200 notifies the base station1 to which the mobile station 100 performed a connection request of anotification (hereinafter, referred to as a connection base stationnotification) which concerns this connection base station (Step S501),and the base station 1 notifies the mobile station 100 of thisconnection base station notification (Step S502). Further, the RNC 200notifies the base station 2 determined as the connection base station ofconnection permission with the mobile station (Step 503), and the basestation 2 notifies the mobile station 100 of the connection permission(Step S504). The base station 2 starts user data communication with themobile station 100 (Step S505).

FIG. 16 is a figure describing a third method for carrying out handoverprocessing of a control signal at a communication stage, and a sequencechart showing an example of operation of a radio communications systemon the occasion when The second base station changes from a radiotransmission suspension state to an active state and hands over a mobilestation which are accommodated the first base station to the second basestation in the first exemplary embodiment. Further, in order to make thedifferences with FIG. 15 clear, in FIG. 16, processing identical withprocessing shown in FIG. 15 is given a step number identical with FIG.15, and in the following description, a description of a step identicalwith FIG. 15 may be partly omitted.

In the third method shown in FIG. 16, processing from Step S30(processing in which mobile station 100 transmits a connection requestwhich concerns an outgoing call and so on (that is, “control signal”) tothe base station 1 in the neighborhood of the base station 2) to StepS43 (processing in which the base station 1 adds the base station 2 tothe measurement cell set) is identical with the second method shown inFIG. 15. The difference between the third method shown in FIG. 16 andthe second method shown in FIG. 15 exists in a point that there aredifferences in the processing thereafter. In the third method, in caseof a handover which does not involve user data communication, the basestation 1 resets a connection request (that is, connection request ofStep S30 in FIG. 16) from the mobile station 100 (Step S600) and startsa connection request procedure to the base station 2 newly (StepsS601-S607). Specifically, the mobile station 100 which received a resetcommand of the connection request performs a reception measurement ofthe pilot signal of each of the base station 1 and the base station 2(Step S601). The mobile station 100 determines a base station to whichtransmits a connection request from the reception measurement result ofthe pilot signal (Step S602). For example, in case the received power ofthe pilot signal of the base station 2 is higher than a threshold value,the mobile station 100 determines the base station 2 as the base stationto which transmits the connection request. The mobile station 100transmits the connection request to the base station 2 (Step S603). Thebase station 2 transmits the connection request from the mobile station100 to the RNC 200 (Step S604). The RNC 200 notifies the base station 2of connection permission with the mobile station 100 (Step S605). Thebase station 2 notifies the mobile station 100 of the connectionpermission (Step S606). The base station 2 starts user datacommunication with the mobile station 100 (Step S607).

In this case, the RNC 200 does not need to request a received powerreport (result report of power measurement using the measurement cellset) of the base stations 1 and 2 to the mobile station 100. And thiscan be realized in such a way as, after the base station 1 receives adirection (Step S42) from the RNC 200 to the effect that it adds thebase station 2 to the measurement cell set of the base station 1, andadds the base station 2 to the measurement cell set (Step S43), tore-perform acquisition of the pilot signal (Step S45) for the secondtime to the mobile station 100, and to perform a connection request onceagain to a base station with high received power.

Further, each of the first to third methods for carrying out handoverprocessing of a control signal at a communication stage described abovecan be applied to all of the first to third exemplary embodiments.

By the way, in the first to third exemplary embodiments mentioned above,a part which adds a cell of a base station which became the active stateto the measurement cell set and gives a direction to a mobile station tomeasure the cell may be omitted. A mobile station may, by receiving thepilot signal autonomously, measuring its power, and receiving anidentification signal of a cell from a control signal of the cell and soon, report the pilot received power together with an identificationnumber of the cell.

Also, in the first to third exemplary embodiments mentioned above,although a base station and a cell are supposed to be those whichcorrespond one to one, in case one base station has a plurality ofcells, control to perform changes to the active state and the radiotransmission suspension state may be performed by a cell unit.

Also, in the first to third exemplary embodiments mentioned above, theRNC 200 is not necessarily an indispensable component. For example, thefirst base station from which handover originates (for example, in caseof the first exemplary embodiment, the base station 1) can be made tofunction as the RNC 200. In that case, the base station 1 and the basestation 2 are connected directly, for example, via a cable communicationnetwork. Accordingly, the base station 2 as the second base station canreceive, for example, “start request” (for example, in case of the firstexemplary embodiment, Step S35 of FIG. 7) which is a direction for ownbase station to change to the active state St_11, directly from otherequipment than the RNC 200, for example, from the base station 1 as thefirst base station.

Also, in the first to third exemplary embodiments mentioned above, thefirst base station (for example, in case of the first exemplaryembodiment, the base station 1) may include a function (for example, incase of the first exemplary embodiment, Step S5 in FIG. 5) in which theRNC 200 compares received power with a threshold value. In that case,the base station 1 may notify the base station 2 via the RNC 200 ofreceived power status. And a function to compare load data with athreshold value of Step S34 in FIG. 7 may be included in the basestation 1. In that case, the base station 1 may request via the RNC 200to start the base station 2.

Also, concerning processing in which the second base station changesfrom the active state St_11 to the radio transmission suspension stateSt_12, in the first to third exemplary embodiments mentioned above, itwas described that the RNC 200 compares a measurement result of themobile station (for example, the mobile station 101 in FIG. 1) whichreceived the pilot signal of the base station 2 and performedmeasurement with a threshold value (Step S5) and transmits thecomparison result concerned to the base station 2 (Step S6). However,the comparison processing mentioned above can be performed by the basestation 2, not by the RNC 200. In that case, for example, the RNC 200which received a received power status confirmation request from thebase station 2 does not perform comparison processing and transmits themeasurement result itself mentioned above (that is, received powerinformation of the pilot signal of the base station 2 in a mobilestation in communication with other base station in the neighborhood ofthe cell 12, for example, the mobile station 101 in communication withthe base station 1 in FIG. 1) to the base station 2. The base station 2compares the measurement result concerned with a predetermined thresholdvalue. Here, the predetermined threshold value may be stored in advancebefore the comparison processing concerned, for example, in apredetermined storage means of the base station 2. Also, the thresholdvalue can be changed dynamically depending on a predetermined condition.

Also, concerning processing in which the second base station (forexample, in case of the first exemplary embodiment, the base station 2)changes from the active state St_11 to the radio transmission suspensionstate St_12, in the first to third exemplary embodiments mentionedabove, it was described that the RNC 200 compares a measurement resultof a mobile station (for example, mobile station 101 in FIG. 1) whichreceived the pilot signal of the base station 2 and performedmeasurement with a threshold value (Step S5), transmits the comparisonresult concerned to the base station 2 (Step S6), and in case themeasurement result mentioned above is below the threshold value, thebase station 2 changes to the radio transmission suspension state St_12(corresponds to processing of Steps S7-S12 of FIG. 5). However, acondition on which the second base station changes to the radiotransmission suspension state St_12 is not limited to the above. Forexample, the RNC 200 which received (Step S4 of FIG. 5) a received powerstatus confirmation request from the base station 2, in case judged thata mobile station (a mobile station to which a control signal of a secondbase station is transmitted) in communication with the base station 1does not exist, sets a comparison result to “measurement result is nomore than a threshold value” forcibly and transmits it to the basestation 2 (corresponds to Step S6 of FIG. 5). The base station 2 whichreceived the comparison result concerned changes to the radiotransmission suspension state St_12 by performing Steps S7-S12 of FIG.5. Thus, by so doing, also in case a mobile station in communicationdoes not exist around the base station 1 other than the mobile stationwhich has ended communication with the base station 2, to make the basestation 2 change to a radio transmission suspension state becomespossible.

Also, according to the first to third exemplary embodiments mentionedabove, although it was described to handover “a mobile station whichperformed a connection request newly” to the first base station from thefirst base station to the second base station, it is also possible tohandover “a mobile station already in communication with a first basestation” from the first base station to the second base station. In thatcase, for example, in case of the first exemplary embodiment, processingis started from Step S32 in FIG. 7. Also, because communication with amobile station is already started, Steps S39-S41 are omitted.Thereafter, processing after Step S42 of FIG. 7 is carried out, and “amobile station already in communication with the first base station” canbe handed over from the first base station to the second base stationsimilar to “a mobile station which performed a connection requestnewly”.

Similarly, although it was described “a mobile station which performed aconnection request newly” to the first base station is handed over fromthe first base station to the second base station, for example, in thefirst exemplary embodiment, “a mobile station which was handed over tothe cell 11 from a cell other than the cell 11” can be handed over fromthe first base station to the second base station. For example, in caseof the first exemplary embodiment, it is possible to replace from StepS30 of FIG. 7 to a handover command from the RNC 200, and to replacefrom a connection establishment report after starting communication toStep S31. Thereafter, processing after Step S32 of FIG. 7 is performed,and “a mobile station which is a mobile station which was handed over tothe cell 11 from a cell other than the cell 11” can be handed over fromthe first base station to the second base station similar to “a mobilestation which performed a connection request newly”.

Also, in the first to third exemplary embodiments mentioned above, inprocessing of Step 5 shown in FIG. 5 (that is, processing in which theRNC 200 compares received power and a threshold value of the second basestation in a mobile station), the threshold value may be changeddepending on load data of the first base station. For example, bysetting up the threshold value so that handover tends to take placeeasier when load is heavy, it becomes easier to get the effect of loadbalancing.

Also, in the first to third exemplary embodiments mentioned above,processing of Step 5 shown in FIG. 5 (that is, processing in which theRNC 200 compares received power and a threshold value of the second basestation in a mobile station) may be performed in the second basestation. In that case, by the second base station 2 requesting receivedpower information to the RNC 200, by the RNC 200 which received therequest transmitting a measurement result of a mobile station to thesecond base station, and by the second base station comparing thetransmitted measurement result of a mobile station with a thresholdvalue, similar operation can be possible. Also, at that time, the RNC200 may transmit from a result with high numerical value amongmeasurement results in a mobile station. By transmitting from a highresult, it becomes possible to complete comparison processing in thesecond base station (processing equivalent to step S5 which shifted to asecond base station) quickly.

Also, in the first to third exemplary embodiments mentioned above, at atime of processing of Step S13 shown in FIG. 5 (that is, processing inwhich the RNC 200 notifies the second base station of receiving statechange information), it is possible for the RNC 200 to inquire load dataof the first base station (the base station 1), and in order to restrictstate change of the second base station (the base station 2) and keepthe active state based on the load state concerned, to reply before StepS12.

Also, in the first to third exemplary embodiments mentioned above,sequence of a start request (for example, in case of the first exemplaryembodiment, processing of Step S35 in FIG. 7: transmission processing ofa start request from the RNC 200 to the base station 2) and connectionpermission (for example, in case of the first exemplary embodiment,processing of Step S39 in FIG. 7: transmission processing of aconnection permission notification with the mobile station 100 from theRNC 200 to the base station 1) does not matter. That is, just afterreporting to the RNC 200 that the base station 1 received a connectionrequest from the mobile station 100, the RNC 200 issues to the basestation 1 connection permission with a mobile station, and after thebase station 1 and the mobile station 100 start to communicate, the basestation 2 may change to the active state St_11.

Also, sequence of the connection permission and an addition direction ofthe measurement cell set (Step S42 in FIG. 7: transmission processing ofa direction to add the base station 2 to the measurement cell set of thebase station 1) does not matter. That is, the direction to add the basestation 2 to the measurement cell set may be before the base station 1and the mobile station 100 start to communicate.

Also, in the first to third exemplary embodiments mentioned above, whena handover is performed, although only received power of the pilotsignal of the base station 2 in a mobile station was used for decisioncriteria of handover (for example, in case of the first exemplaryembodiment, Step S48 in FIG. 7), it is not limited to that. Handover maybe carried out after comparison with received power of the pilot signalof the base station 1 in communication is performed. By comparing withthe received power of the pilot signal of the base station 1, it becomespossible to prevent communication quality to degrade with handover.

Also, in the first to third exemplary embodiments mentioned above, itwas described that the first base station (the base station 1 and thebase station 4) and the second base station (the base station 2) areloaded with exclusive functions respectively, and are special purposemachines conforming to each purpose. However, by loading one basestation with the function which only the other base station is loaded,the function of the first base station and the second base station is tobe shared. Accordingly, by so doing, it is possible for the second basestation to carry a function of the first base station which wasdescribed above, and oppositely, for the first base station to carry afunction of the second base station which was described above.

Also, in the first to third exemplary embodiments mentioned above, itwas described that the first base station and the second base stationare controlled by special purpose hardware. However, it is also possibleto make these the first base station and the second base station becontrolled based on a control program by a computer circuit (CPU(Central Processing Unit), for example) which is not shown, and operate.In that case, these control programs are stored in a storage medium(such as ROM (Read Only Memory) or hard disk, for example) inside eachof base station mentioned above or in an external storage medium (suchas removable media or removable disk, for example), and are read out bythe computer circuit mentioned above and carried out.

According to the first to third exemplary embodiments described above,it was described that the second base station makes an operation stateof own base station change to the active state St_11 (in other words,start transmission of the pilot signal with predetermined power) by astart request received from other equipment (for example, the RNC 200 orthe first base station). Here, the start request is a start request tothe second base station based on a connection request by a mobilestation to the first base station. However, the base station 2 canchange to the active state St_11 autonomously not based on a directionfrom other equipment.

For example, the second base station receives a transmission signal,from a mobile station to the first base station, and depending on areception condition of the transmission signal concerned, can alsorecognize starting of communication between the first base station andthe mobile station. In this case, the second base station can recognizestarting of communication between the first base station and the mobilestation by measuring a received power of the transmission signal of themobile station in upstream band to the first base station.

Also, the second base station can recognize starting of communicationbetween the first base station and a mobile station by detecting anoutgoing call of the mobile station to the first base station.

Also, the second base station can recognize starting of communicationbetween the first base station and a mobile station by judging, usingradio resources allocation information which the first base stationuses, whether a transmission signal of the mobile station to the firstbase station is included in a signal which the second base stationreceived.

Further, radio resources allocation information is either of time slotinformation assigned to each mobile station, spreading code informationassigned to each mobile station, or frequency band information assignedto each mobile station; or a combination of two or more of theinformation.

However, in this case, in case information which concerns a decisionabout a realization of a predetermined condition (load or communicationquality of the first base station to become a predetermined state)cannot be collected by the second base station only, the information isreceived from other equipment (the RNC 200 or the first base station).In other words, the second base station is characterized by including ameans which receives a transmission signal from a mobile station, and ameans which, depending on a reception condition of the transmissionsignal concerned, starts transmission of the pilot signal withpredetermined power when the predetermined condition is satisfied.

Also, a cell configuration of the first and the second base station ofthe radio communications system of the first to third exemplaryembodiments described above can be made a hierarchy cell (also referredto as an overlap cell) structure. For example, it is also possible tomake the first base station a macro cell, and the second base station asmall cell (micro cell, nano cell or femto cell, for example) whoseentire cover area is included in the cover area of the macro cell.

Further, as an example of handover of the first to third exemplaryembodiments mentioned above, although a method to handover a mobilestation after it is connected concurrently with two or more basestations (soft handover) is used, the soft handover is notindispensable. Form of handover which changes a base station of aconnection destination of a mobile station does not matter. For example,a method, where timing to change connection is estimated at basestations of a handover source and a handover destination, and handoveris performed without waiting for a time to connect concurrently (hardhandover); can be used.

Although the present invention has been described with reference toexemplary embodiments above, the present invention is not limited to theexemplary embodiments mentioned above. Various changes which a personskilled in the art can understand within the scope of the presentinvention may be performed in the composition of the present inventionand details.

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2008-275915, filed on Oct. 27, 2008, thedisclosure of which is incorporated herein in its entirety by reference.

1.-44. (canceled)
 45. A radio communications system comprising: a firstbase station; a second base station; and at least one mobile stationwhich can communicate with said first base station and said second basestation, wherein said second base station starts transmission of acontrol signal with predetermined power when said first base stationreceives a connection request transmitted from a mobile station and apredetermined condition is satisfied.
 46. The radio communicationssystem according to claim 45, wherein said second base station startstransmission of the control signal with predetermined power, when acondition, as said predetermined condition, that load information ofsaid first base station becomes larger than a predetermined thresholdvalue is satisfied.
 47. The radio communications system according toclaim 45, wherein said second base station starts transmission of thecontrol signal with predetermined power, when a condition, as saidpredetermined condition, that a location of a mobile station whichstarts communication with said first base station is within a rangewhere it can communicate with said second base station is satisfied. 48.The radio communications system according to claim 45, wherein saidsecond base station starts transmission of the control signal withpredetermined power, when a condition, as said predetermined condition,that more than predetermined number of mobile stations which startcommunication with said first base station exist within a range wherethey can communicate with said second base station is satisfied.
 49. Theradio communications system according to claim 45, wherein correspondingrelation between said mobile station and predetermined said second basestation is provided beforehand; and said second base station having saidcorresponding relation with said mobile station starts transmission ofthe control signal with predetermined power, when a condition, as saidpredetermined condition, that a second base station having saidcorresponding relation with a mobile station which starts communicationwith said first base station exists is satisfied.
 50. The radiocommunications system according to claim 45, wherein correspondingrelation between call of said mobile station and said predeterminedsecond base station; and said second base station having saidcorresponding relation with said call starts transmission of the controlsignal with predetermined power, in case a condition, as saidpredetermined condition, that said second base station having saidcorresponding relation with a call of a mobile station which startscommunication with said first base station exists is satisfied.
 51. Theradio communications system according to claim 45, wherein said secondbase station changes a base station which communicates with said mobilestation from said first base station to said second base station basedon a change command transmitted from said first base station dependingon a reception condition of said control signal in said mobile station.52. The radio communications system according to claim 45, wherein saidsecond base station acquires via said first base station reception powerinformation which is obtained by a mobile station which communicateswith said first base station by receiving a control signal which saidsecond base station transmits, and suspends transmission of said controlsignal when the reception power information is lower than a thresholdvalue for a fixed time.
 53. The radio communications system according toclaim 45, wherein said second base station suspends transmission of saidcontrol signal when a command to change a base station whichcommunicates with said mobile station from said first base station tosaid second base station is not received after starting transmission ofsaid control signal.